The present invention relates to an optical filter comprising a Bragg grating formed in an optical fiber, to a method and apparatus for manufacturing such a filter, and to a fiber holder and phase mask used in the manufacturing process.
In-fiber Bragg gratings, also known simply as fiber Bragg gratings or FBGs, are useful in the field of optical communication as optical filters for such purposes as wavelength-division multiplexing and dispersion compensation. U.S. Pat. No. 5,367,588 describes a method of manufacturing an in-fiber Bragg grating by exposing a photosensitive optical fiber to ultraviolet light through a phase grating mask. The phase grating mask comprises a quartz glass plate, which is transparent to ultraviolet light, having a periodic relief pattern of parallel corrugations on one surface. The corrugations have the form of, for example, parallel channels with a rectangular cross section. Diffraction in the phase mask modulates the intensity of the emerging ultraviolet light with a periodicity determined by the grating spacing or pitch.
The photosensitive optical fiber is placed in contact or near-contact with the phase grating mask, in a direction orthogonal to the corrugations. Exposure to the ultraviolet light changes the refractive index of the core of the fiber, imprinting an index modulation in the fiber core with the same periodicity as that of the phase grating mask. This index modulation constitutes the Bragg grating.
A chirped Bragg grating can be formed by modulating the grating pitch of the phase grating mask. An apodized Bragg grating can be formed by modulating the strength of the ultraviolet light along the length of the optical fiber.
The phase grating mask can be fabricated by reactive ion etching of a fused quartz substrate, as described, for example, on page 567 of Electronics Letters, Vol. 29, No. 6 (Mar. 18, 1993).
Filter performance parameters such as the reflection bandwidth and the top flatness of the reflection spectrum are known to depend on the length of the imprinted grating. When an in-fiber Bragg grating is used for dispersion compensation, for example, the reflection bandwidth .DELTA..lambda. is given by the following formula, in which L is the length of the Bragg grating, c is the speed of light, and D is the dispersion value. EQU .DELTA..lambda.=2L/(cD)
This formula indicates that for a given dispersion D, the reflection bandwidth .DELTA..lambda. increases in proportion to the grating length L.
Long in-fiber Bragg gratings are not easily fabricated with a phase grating mask of the type described above, however, because the size of the phase grating mask is limited by the need to form the phase grating mask itself in a vacuum chamber. A step-and-repeat process can be carried out by moving the fiber past the phase grating mask, but this process is time-consuming and requires extremely accurate alignment from one step to the next. For these reasons, the length of in-fiber Bragg gratings formed by use of conventional phase grating masks has been limited to a maximum of about one hundred millimeters (100 mm).
The limited length of the conventional phase grating mask is thus an obstacle to the attainment of wide reflection bandwidths and other desirable filter characteristics. The limited length is also an obstacle to effective apodization of the in-fiber Bragg grating.
A further obstacle to the use of long in-fiber Bragg gratings is the need to package the fiber containing the grating in such a way as to protect the grating from temperature variations and other external effects. Conventional packaging processes cannot easily be applied to long lengths of fiber.